In the field of electricity generation, it is well known to provide a generator coupled to a fluid driven turbine such as those found in oil, gas, coal and nuclear power stations. Conventional generators comprise a rotor having an iron core with a number of current-carrying coils wound on the core, and an iron-cored stator carrying a winding. A magnetic field is generated by passing a current along the rotor coils such that, on rotation of the rotor, a current is induced in the coils of the stator winding.
In recent years, significant research has been conducted Worldwide into sustainable electricity generation methods, including wind, wave and tidal power generation. Through this research, wind machines have been developed which comprise a prime mover in the form of a large diameter rotor having a number of rotor blades, mounted on a rotor shaft, which is in turn coupled to a power generator. The wind machines include rotary generators which operate in a similar fashion to the large-scale generators found in power stations and described above, save that a gear mechanism is required to enable the generator to operate at the relatively low velocity and high output torque of the rotor.
Use of such gear mechanisms is generally undesired as there are a number of significant disadvantages. In particular, the gear mechanisms are relatively large and heavy, greatly increasing the weight of the unit provided in the nascelle at the top of the wind turbine tower. Additionally, provision of a gear mechanism between the output shaft of the turbine rotor and the input shaft of the generator reduces the efficiency of the machine. Furthermore, these gear mechanisms have been found to be surprisingly unreliable under typical wind turbine operating conditions. The main cause for this is the constant variation in the operating speed and torque transmitted through the gear mechanisms, which is due to fluctuations in wind velocity.
Similar problems have been experienced in power generation systems using wave and tidal forces, where the prime movers of the systems operate at even lower rotational or cycling velocities, and hence at still higher torques or thrust forces.
To address these problems, different types of power generators have been developed which are designed for low speed, high torque operation, for direct connection to, for example, the rotor of a wind machine. These generators are significantly larger than prior generators connected through a gearbox.
A significant disadvantage to such directly coupled generators is that the iron core of the generator (which provides an active magnetic function) and structural support for the iron core (which provides a purely mechanical function of maintaining a physical air gap between stationary and rotating/reciprocating components) is very large, and thus the overall mass is high. Where the generator is provided in a wind machine, mounted in a nascelle at a top of a tower of the machine, significant problems are encountered in the requirement to strengthen the tower to support the additional weight of the generator; in raising and locating the nascelle on the tower; and in removing the generator/nascelle for maintenance.
Similar problems are encountered in other types of sustainable electricity generation, which may utilise a direct drive of the type described above (such as in wave machines and tidal current machines), and which may utilise a rotary or linear type generator.
Furthermore, whilst the mass of such direct drive generators has been found to be a particular problem in low speed/high torque output sustainable electricity generation machines, it will be appreciated that prior generators such as those used in conventional power stations are also of a significant mass, due in the main to the mass of the iron cores used. In the case of such conventional generators, however, the iron core provides both a structural function as well as a magnetic flow path, which is in contrast to known direct drive generators in which two separate components provide magnetic and structural functions.
It is therefore amongst the objects of embodiments of the present invention to obviate or mitigate at least one of the foregoing disadvantages.